1. Field of the Invention
The present invention relates to a color CRT (cathode ray tube), and more particularly, to a mask frame assembly in which a creep deformation due to a thermal process of a mask receiving a tension is prevented and a thermal compensation characteristic during the operation of a CRT is improved, and a color CRT adopting the same.
2. Description of the Related Art
In a typical color CRT, three electron beams emitted from an electron gun pass through electron beam passing holes of a mask having a color selection function and land on red, green and blue fluorescent substances of a fluorescent film formed on a screen surface of a panel to excite the fluorescent substances, thus forming an image.
In the above color CRT forming an image, the mask having a color selection function is largely divided into a dot mask, which is used in computer monitors, and a slot mask (or a slit mask), which is used in televisions.
Many studies have been made about a tension mask, which is one type of slot mask that is supported such that a tension is applied by a frame, considering a flat screen surface, to correct distortion of an image and increase a view angle of a screen. A frame and a mask frame assembly, where a mask is supported such that a tension is applied by the frame, are installed in a panel of a color CRT. FIGS. 1 and 2 show an example of such a color CRT.
Referring to the drawings, a color CRT includes a panel 13 having a flat screen surface 12. A fluorescent film 11 is formed on the flat screen surface 12. A tension mask frame assembly 20 is suspended at the inner surface of the panel 13. A funnel 15 is coupled to the panel 13 and forms a seal in which an electron gun 16 is installed in a neck portion 14 of the funnel 15. A deflection yoke 17 is installed at a cone portion of the funnel 15.
The tension mask frame assembly 20 includes a tension mask 22, where a plurality of slots 21 are formed, a pair of support members 23 to support one pair of opposite edges of the tension mask 22, and a pair of elastic members 24 to support end portions of each of the support members 23 so as to apply a tension to the tension mask 22. The mask frame assembly 20 is supported by spring supporters 25 at the support members 23 and the elastic members 24 and is suspended in the panel 13 by a hook spring 26 coupled to a stud pin (not shown) installed at the inner surface of the panel 13.
In the tension mask frame assembly 20 having the above structure, as the spring supporter 25 is heated by electron beams not passing through the slots 21, the spring supporter 25, which is formed of a bimetal, is deformed and moves the tension mask frame assembly 20 toward the panel 13. Thus, mis-landing of electron beams due to the thermal expansion of the tension mask frame assembly 20 is corrected. An example of the above tension mask frame assembly is disclosed in Japanese Patent Publication No. 8-124489.
Referring to FIGS. 3 and 4, a spring supporter 31, which is formed of a bimetal, is fixed to the outer circumferential surface of the frame. A spring 32, which has a coupling hole 32a to be coupled to a stud pin 13a installed on the inner surface of the panel 13, is fixed at one end portion of the spring support 31. The spring 32 is formed of a single material.
In a color CRT including a fixing structure of the tension mask frame assembly 20, as shown in FIGS. 1 and 3, after being deflected by the deflection yoke 17, the electron beam emitted from the electron gun 16 passes the electron beam passing holes of the tension mask 33 and lands on a fluorescent film to excite fluorescent substance coated thereon. In this process, part (15 through 25%) of the electron beam emitted from the electron gun 16 passes through the electron beam passing holes of the tension mask 33. The remaining part of the electron beam not passing through the electron beam passing holes hits the tension mask 33 and heats it. Thus, the tension mask 33 and the frame 34 supporting the tension mask 33 are thermally extended by being heated by the electron beam, that is, thermions.
The thermal expansion of the tension mask 33 and the frame 34 results in a displacement of the electron beam passing holes of the tension mask 33, which causes mis-landing of the electron beam onto the fluorescent film. The mis-landing of the electron beam is corrected as follows using the device shown in FIG. 4. The spring supporter 31 is formed of a bimetal, and when thermally deformed, the tension mask frame assembly 30 is moved toward the panel 13 so that the electron beam passing holes moved due to the thermal expansion of the tension mask 33 are positioned fitting to the trace of the electron beam. Thus, the thermal expansion of the tension mask frame assembly 30 is corrected.
However, as the spring supporter 31 thermally expands, the tension mask frame assembly 30 has a rotational component. Since the rotational component of the tension mask frame assembly 30 generates the mis-landing of the electron beam, the quality of an image deteriorates. Also, since the spring supporter 31 is formed of a bimetal, the manufacturing cost increases.
In the meantime, the tension mask frame assembly 30 undergoes an annealing process to remove stress due to welding the support members and the elastic members during the manufacturing process. In the annealing process, the tension mask frame assembly 30 is heated up to around 500° C. Here, due to a difference between the amount of thermal expansion of the frame 34 and the amount of thermal expansion of the mask 33, the mask 33 is plastically deformed so that a tension decreases (by 50% of a tension before the annealing process). That is, as the mask frame assembly 30 is heated, a difference in the amount of thermal expansion is generated because the heat capacity of the mask 33 is less than that of the frame 34. The difference in the amount of thermal expansion acts as an additional tension to the tension mask 33 supported at the support member so that the tension of the tension mask 33 decreases after the annealing process. The decrease in the tension of the tension mask 33 causes a howling phenomenon when the tension mask 33 is installed at a color CRT and used therein, or produces an electron beam drift phenomenon due to the thermal deformation of the mask.
To solve the above problem, a mask frame assembly to prevent the operation of the amount of expansion of the frame in a direction in which the tension acts on the mask is disclosed in U.S. Pat. No. 5,111,107. The disclosed mask frame assembly is shown in FIG. 5. As shown in the drawing, the mask frame assembly 40 includes support bars 41 installed at the opposite positions, elastic support members 42 and 42 installed between the support bars 41 to support the support bars 41, a mask 43 supported by the support bars 41, and metal members 44 installed at the surfaces of the elastic support members 42 opposite to the surfaces facing the mask 43 and having a thermal expansion coefficient greater than that of the elastic support members 42.
In the above mask frame assembly 40, a tension of the mask 43 is lowered in spite of the attachment of the metal members 44. Also, the effect of the metal members 44 varies according to the distribution of the tension.
A color CRT having a structure of a mask frame assembly to prevent reduction of a tension of a mask during the annealing process is disclosed in Japanese Patent Publication No. 11-317176. The disclosed color CRT has a color selection electrode in which a grid is suspended at a frame having a pair of support bodies facing each other and a pair of elastic support members installed between the support bodies. In the disclosed color CRT, a control member having a thermal expansion coefficient that is low at a lower temperature and is high in a high temperature area, compared to a thermal expansion coefficient of the elastic support bodies, is fixed at the surface opposite to the grid of the elastic support members, or a control member having the opposite characteristic is fixed at the elastic support member at the side opposite to the grid. Since a color selection apparatus of the color CRT having the above structure is merely the control member using a difference in the thermal expansion coefficient which is attached to the elastic support members, the above problems are fundamentally solved.